Titanium dioxide pigments are used in connection with coating formulations (including paint and ink formulations), paper compositions, polymer compositions and other products. Such pigments are generally produced in powder form with specific properties and characteristics depending on the final application.
The titanium dioxide pigments can be manufactured by either the sulfate process or the chloride process.
In the sulfate process for manufacturing titanium dioxide, a titanium slag ore is dissolved in sulfuric acid to form titanyl sulfate. The titanyl sulfate is then hydrolyzed to form hydrous titanium dioxide. The hydrated titanium dioxide is heated in a calciner to grow titanium dioxide crystals to pigmentary dimensions.
In the chloride process for manufacturing titanium dioxide, a dry titanium dioxide ore is fed into a chlorinator together with coke and chlorine to produce a gaseous titanium halide (such as titanium tetrachloride). The produced titanium halide is purified and oxidized in a specially designed reactor at a high temperature to produce titanium dioxide particles having a desired particle size. Aluminum chloride is typically added to the titanium halide in the oxidation reactor to facilitate rutile formation and control particle size. The titanium dioxide and gaseous reaction products are then cooled, and the titanium dioxide particles are recovered.
Whether produced by the sulfate process or the chloride process, the produced titanium dioxide particles typically undergo further processing steps. For example, further processing steps commonly utilized include: (a) dispersing the particles in an aqueous medium to form a pigment slurry (a dispersing agent such as a polyphosphate is typically used); (b) wet milling the resulting pigment slurry to achieve titanium dioxide particles having a predetermined particle size; (c) precipitating one or more hydrous metal oxide inorganic materials such as silica ceria, zirconia and/or alumina onto the particle surfaces of the wet milled titanium dioxide slurry; (d) flocculating the treated particles and recovering the inorganic oxide-treated titanium dioxide particles from the aqueous slurry by filtration; (e) washing the filtered particles to remove residual salts and impurities thereon; (f) drying the washed filtered particles to provide a dry titanium dioxide pigment powder; and (g) fluid-energy milling the dried pigment. Any agglomerates formed during the above described steps are typically strengthened during the drying stage and usually require energy intensive milling to break down the agglomerates to a desired particle size.
In the pigment manufacturing industry, the fluid-energy milling step is often carried out using either compressed air, steam, or inert gases. Although other processing steps have been combined with fluid-energy milling in an attempt to deagglomerate the pigment at reduced costs, the abrasiveness of such steps can adversely impact the coatings on the pigment.